At least 16 Palestinianswere killed and hundreds injured by Israeli security forces confronting one of the largest Palestinian demonstrations along the Israel-Gaza border in recent years, Gaza medical officials have said.

Key points:

Tens of thousands of Gazans, including families with children, gather along the border as clashes unfold

Israeli troops fire live bullets and tear gas at Palestinians on the other side of the fence

Protesters are demanding that Palestinian refugees be allowed the right of return to towns and villages from which their families fled in 1948

Tens of thousands of Palestinians, pressing for a right of return for refugees to what is now Israel, gathered at five locations along the fenced 65 kilometre frontier where tents were erected for a planned six-week protest, local officials said. The Israeli military estimate was 30,000.

The United Nations Security Council was briefed on the violence in Gaza on Friday at the request of Kuwait.

Families brought their children to the encampments just a few hundred metres from the Israeli security barrier with the Hamas Islamist-run enclave, and football fields were marked in the sand and scout bands played.

But as the day wore on, hundreds of Palestinian youths ignored calls from the organisers and the Israeli military to stay away from the frontier, where Israeli soldiers across the border kept watch from dirt mound embankments.

The military said its troops had used "riot dispersal means and firing towards main instigators".

Some of the demonstrators were "rolling burning tires and hurling stones" at the border fence and at soldiers.

Two Palestinians were killed by tank fire, the Gaza Health Ministry said.

Live fire was used only against people trying to sabotage the border security fence and at least two of the dead were Hamas operatives, an Israeli military official said.

Gaza health officials said one of the dead was aged 16 and at least 400 people were wounded by live gunfire, while others were struck by rubber bullets or treated for tear gas inhalation.

Palestinian President Mahmoud Abbas said in a statement that Israel was responsible for the violence and declared Saturday a national day of mourning.

Israel's chief military spokesman said if violence drags on along the Gaza border, Israel will expand its reaction to strike the militants behind it.

Brigadier General Ronen Manelis said on Saturday the military has thus far restricted its response to those trying to breach its border but if attacks continue it will go after militants "in other places too".

UN Secretary-General Antonio Guterres called for an independent, transparent investigation and appealed "to those concerned to refrain from any act that could lead to further casualties and in particular any measures that could place civilians in harm's way", his spokesman said in a statement.

A senior UN official told the UN Security Council there are fears the situation in Gaza "might deteriorate in the coming days".

The United States, a close Israel ally, told the council it was "deeply saddened" by the loss of life.

"We urge those involved to take steps to lower tensions and reduce the risk of new clashes … Bad actors who use protests as a cover to incite violence endanger innocent lives," a US diplomat said.

Protesters call for 'right of return'

The protest marks "Land Day", an annual Palestinian commemoration of the deaths of six Arab citizens of Israel killed by Israeli security forces during demonstrations over government land confiscations in northern Israel in 1976.

The demonstrators are demanding that Palestinian refugees be allowed the right of return to towns and villages which their families fled from, or were driven out of, when the state of Israel was created in 1948.

Israeli police announced heightened security measures nationwide from before the start of Passover on Friday at sundown and through the Jewish and Christian holidays including the eastern Orthodox celebrations of Easter next weekend.

An Israeli police spokesman said thousands of officers would be deployed, including "special patrol units" in all cities.

Eighty-year-old Mansi Nassar walked towards the border with the aid of his cane ignoring calls to remain 700 metres from the barrier.

"I was born in Beit Darras inside Palestine and I will accept no less than returning to it," he said, referring to his former home village just south of the modern Israeli city of Ashdod. The village no longer exists.

Israel rules out any right of return, fearing an influx of Arabs that would wipe out its Jewish majority. Israel argues that refugees should resettle in a future state the Palestinians seek in the Israeli-occupied West Bank and Gaza.

The protest is scheduled to culminate on May 15, the day Palestinians mark what they call the "Nakba", or "Catastrophe" when the Israeli state was created.

The protest organisers include Hamas, the armed Islamist movement that dominates Gaza and representatives of other Palestinian factions.

Hamas leader Ismail Haniyeh spoke at one tent encampment on Friday, saying that Gazans were demanding a "return to Palestine, all of Palestine. No concessions and no recognition of the Zionist entity [Israel] in any inch of the land".

With feelings raw on both sides, however, Hamas urged protesters to adhere to the "peaceful nature" of the march.

An Israeli Foreign Ministry statement said the protest was being orchestrated in Gaza by Hamas in a deliberate attempt to provoke a violent confrontation.